The long term goals of this proposed research in vascular physiology are: to define the total geometry and functional geometry of selected parts of the vascular system: to analyze that geometry by application of theoretical mechanics; to test, when applicable, theory with physical laboratory models, and to validate by experimentation appropriate physiological mechanisms derived from a theoretical analysis. It is expected that this approach will lead to increased understanding of physiological control mechanisms which regulate blood flow and its distribution. Within the above framework, an emphasis will be placed on selected microvasculatures, especially the pulmonary interalveolar bed, its elastic response and the structures that govern it, on microstructural analysis of the arterial wall, and erythrocyte- capillary diameter relationships. The experimental methods of silicone elastomer microvascular casting and quick freezing are directed to arresting the geometric functional state of the microvasculature. Structure will be examined at various levels, including electron microscopy.